I've been using an OD-20 for almost a year now, and have been very happy with the Boss sounds, of course Boss should be able to nail their own sounds without too much trouble.
But until a few nights ago I never had a chance to A/B the OD-20 with any of the other manufacturer's models. As it happened, shoudek came over and brought his Fulltone Fulldrive with him. We put them both through the same amp and same guitar, and within about 5 minutes were able to emulate the Fulltone almost exactly. We had some other things to do so we had to quit goofing off and get busy, but I'm very confident that with a few more minutes the tone would've been nailed.
I still want to try it against a Tube Screamer, Rat, and a few of the other heavyweights. But even if they only get close, who can argue with a 1/2 dozen classic Boss pedals and a Fulldrive? Especially when you consider the cost.
This is the first modeling anything I've ever owned that actually does what they say. When I bought it, I was just looking for a few available distortions that I could call up. I never really thought it would perform this well. For $159, I'm thrilled.